Within the food packaging industry, packages have been used for a long time which are formed from a web or a blank of packaging material comprising different layers of paper or board, liquid barriers of for example polymers and gas barriers of for example thin films of aluminium. To extend the shelf-life of the products being packed, it has been known to sterilize the web before the forming and filling operations, and to sterilize the partly formed packages (ready-to-fill packages, RTF packages) before the filling operation. Depending on the length of shelf-life desired and whether the distribution and storage is made in at a chilled or ambient temperature, different levels of sterilization can be chosen. One way of sterilizing a web involves chemical sterilization using, for example, a bath of hydrogen peroxide. Similarly, a ready-to-fill package can be sterilized by hydrogen peroxide, preferably in a gas phase.
Another way to sterilize packaging material is to irradiate it by electrons emitted from an electron beam emitting device such as, for example, an electron beam generator. Such sterilization of a web of packaging material is disclosed in International Application Publication Nos. WO 2004/110868 and WO 2004/110869. Similar irradiation of ready-to-fill packages is disclosed in International Application Publication No. WO 2005/002973. The disclosure in each of the three international application publications mentioned above is hereby incorporated by reference.
To provide on-line control of the intensity of the electron beam, and to monitor uniformity variations, electron sensors are used for dose irradiation measurement. A signal from the sensor is analyzed and fed back into an electron beam control system as a feedback control signal. In the sterilization of packaging material, such sensor feedback can be used to assure a sufficient level of sterilization.
One kind of existing sensor for measuring electron beam intensity, based on direct measuring methods, uses a conductor placed within a vacuum chamber. The vacuum chamber is used to provide isolation from the surrounding environment. Because vacuum-based sensors can be relatively large, they are located at positions outside the direct electron beam path to avoid shadowing of target objects. Shadowing can, for example, preclude proper irradiation (and thus, proper sterilization) of packaging material. Therefore, these sensors rely on secondary information from a periphery of the beam, or information from secondary irradiation, to provide a measurement.
In operation, electrons from the electron beam which have sufficient energy will penetrate a window, such as a titanium (Ti) window of the vacuum chamber and be absorbed by the conductor. The absorbed electrons establish a current in the conductor. The magnitude of this current is a measure of the number of electrons penetrating the window of the vacuum chamber. This current provides a measure of the intensity of the electron beam at the sensor position.
A known electron beam sensor that has a vacuum chamber with a protective coating, and an electrode representing a signal wire inside the chamber, is described in U.S. Application Publication No. 2004/0119024. The chamber walls are used to maintain a vacuum volume around the electrode. The vacuum chamber has a window accurately aligned with the electrode to sense the electron beam density. The sensor is configured for placement at a location, relative to a moving article being irradiated, opposite the electron beam generator for sensing secondary irradiation.
A similar electron beam sensor is described in International Application Publication No. WO 2004/061890. In one embodiment of this sensor, the vacuum chamber is removed and the electrode is provided with an insulating layer or film. The insulating layer is provided to avoid influence from electrostatic fields and plasma electrons created by the electron beam from substantially influencing the electrode output.
U.S. Pat. No. 6,657,212 describes an electron beam irradiation processing device wherein an insulating film is provided on a conductor, such as a stainless steel conductor, of a current detection unit placed outside a window of an electron beam tube. A current measuring unit includes a current meter that measures the current detected. This patent describes advantages of a ceramic coated detector.
Another type of sensor is described in U.S. Application Publication No. 2007/0114432 filed by the assignee. The disclosed sensor comprises a conducting wire and an isolating shield shielding off at least a portion of the conducting wire from plasma exposure. The plasma shield also comprises an outer conductive layer connected to ground potential for absorbing the plasma. The detector is small and may be placed outside the electron exit window in front of the electron beam. By adding several detectors and distributing them across the electron exit window, multiple measuring points are achieved resulting in a dose mapping of the electron beam.
U.S. Application Publication No. 2007/0090303, also filed by the assignee, describes a multilayer detector which can be used for sensing an electron beam. The detector comprises a conductive wire which is isolated from the surroundings by a thin insulating material. On top of the insulating material a layer of conducting material is deposited, which is connected to a ground potential. Only electrons from the electron beam are capable of penetrating the outer layers to be absorbed by the conducting wire. The outer conducting layer absorbs plasma. The detector is small and may be placed outside the electron exit window in front of the electron beam. By adding several detectors and distribute them across the electron exit window, multiple measuring points are achieved resulting in a dose mapping of the electron beam.
In Swedish Patent Application No. 0502384-1, filed by the assignee, a further sensor is described. The sensor comprises a conductor and an insulating housing. The housing is attached to the electron exit window of the electron beam generator and forms a closed chamber together with said window. The conductor is located in the chamber and is thereby shielded from plasma.